In recent years, in an electronic endoscope apparatus using a solid state image sensor, attention is attracted to spectroscopic imaging combined with a narrow band pass filter, that is, a narrow band filter incorporated electronic endoscope apparatus (Narrow Band Imaging-NBI) based on a prediction of a spectroscopic reflectance in the digesting organ (stomach mucosa or the like). According to the apparatus, three band pass filters of narrow (wavelength) bands are provided in place of a rotating filter of R (red), G (green), B (blue) of a face sequential type, and a spectral image is formed by successively outputting illuminating light by way of the narrow band path filters and processing three signals provided by the illuminating light while changing respective weights thereof similar to a case of R, G, B (RGB) signals. According to the spectral image, in the digesting organ of the stomach, the large intestine or the like, a fine structure which cannot be provided in a background art is extracted.
Meanwhile, it has been proposed to form a spectral image by operation processing based on the image signal provided by white light not by the face sequential type using the narrow band pass filters but by a simultaneous type for arranging a color filter of a small mosaic to a solid state image sensor as shown by JP-A-2003-93336 and Tokyo University Printing Association Foundation ‘Analysis and Evaluation of Digital Color Image’ by MIYAKE, yoichi (P148 through P153). According thereto, a relationship between respective color sensitivity characteristics of RGB which are formed into numerical value data and a spectroscopic characteristic of a specific narrow band pass filter which is formed into numerical value data is calculated as matrix data (coefficient set) and by operation of the matrix data and RGB signals, a spectral image signal provided by way of the narrow band pass filter is pseudonically provided. When the spectral image is formed by such an operation, it is not necessary to prepare a plurality of filters in correspondence with a desired band pass region, an interchanging arrangement thereof is dispensed with and therefore, large-sized formation of the apparatus is avoided and low cost formation thereof can be achieved.
Meanwhile, according to a spectral image extracting a specific fine structure or the like of an object, it is necessary to select a preferable wavelength region or adjusting the selected wavelength region and there is a case in which a necessary and sufficient spectral image cannot be provided in inspection by an endoscope in a limited time period. Further, it is necessary to observe and diagnose the spectral image in details by comparing with a normal color image and when a spectral image having an arbitrary wavelength region can be formed and displayed after inspection by the endoscope, an apparatus having an excellent way of use can be provided.
Further, in the operation processing of the spectral image by the endoscope apparatus, for example, color image signals of RGB constituting a basis thereof differs by a spectroscopic sensitivity characteristic including a kind of a color filter of an imaging element (solid state image sensor or the like), a kind of a light source, a spectroscopic sensitivity characteristic of an optical system member of the endoscope of a light guide or the like to pose a problem that such differences in the spectroscopic characteristics of the endoscope of the light source effect an influence on reproducibility on the same wavelength region. That is, there are CCDs constituting solid state image sensors of a complementally color type having color filters of Mg, Ye, Cy, G and a primary color type having color filters of RGB, further, even in the CCD of the same kind, the spectroscopic sensitivity characteristic differs by an individual difference. FIG. 9 shows an example of spectroscopic sensitivity characteristics of color filters of primary color type CCD, respective spectroscopic sensitivities of the R, G, B color filters differ by an individual difference of CCD, in an operation processing using a single matrix data, the difference in the spectroscopic characteristics is reflected to an operation result and the spectral image having reproducibility cannot be provided.
Further, a spectroscopic characteristic of illuminating light differs by an aperture amount of a diaphragm blade in the light source apparatus owing to chromatic aberration of lenses, there is constituted a characteristic in which the more the light amount is reduced, the more the red color component is gradually cut from a long wavelength side to pose a problem that the reproducibility of the spectral image is deteriorated even by the spectroscopic characteristic of the illuminating light.